The present invention relates in general to sheet-fed printing machines and in particular to a driving mechanism for groups of adjustable sheet-gripping elements arranged in a rotary sheet transfer cylinder of a sheet-fed printing machine which is switchable for printing in one of two operational modes such as printing on one side of the sheet only, that is the first form mode, or printing on both sides of the sheet when the sheet is first printed on one side and thereupon is turned over around its rear edge and printed on its back.
Driving mechanisms for groups of gripping elements in a sheet transfer cylinder of sheet-fed rotary printing machines in which the desired movement of the gripping elements is derived from sets of cam drives, are known from prior art. For example, German patent No. 1,107,266 discloses an arrangement in which the movement of swing shafts is controlled by a cam having a cam shaft supporting an arm with a cam follower which rolls on the inner side of the cam and transmits its motion to a toothed segment, wherefrom the movement is transmitted to a cam gear mounted on the swing shaft. The cam is arranged at one side of the transfer cylinder. The movement of grippers is also controlled by a cam arranged at the opposite side of the machine.
In another embodiment of prior-art driving mechanisms of this type, such as disclosed in DL patent No. 59 799, the control of swinging movement is accomplished by means of a roll following the outer surface of a cam.
A common feature of all known driving mechanisms for groups of functional elements in sheet-guiding cylinders, particularly in turnover cylinders, is the fact that the control cams are arranged on a side wall of the printing machine concentrically relative to the axis of rotation of the sheet-guiding cylinder. As a consequence of this concentric arrangement of the cams, the size of the latter has to be designed relatively large and the cams must be mounted on special holders.
Another disadvantage of this prior-art arrangement resides in the fact that, during the assembly of the driving mechanisms for turnover cylinders, the mounting of such cams and cam holders can be effected only after the insertion and angular displacement of the shafts of the turnover cylinder into its bearing openings. For this reason, the cams and the cam holders as a rule have to be constructed of several parts and consequently high manufacturing and installation costs result.
In switching over the mode of operation of the machine, that is, in adjusting the machine from printing on one side only to printing on both sides of the sheet, and vice versa, it is necessary that the functional groups on the sheet transfer cylinders be adjusted also to the selected operational mode, that is, some of the functional groups must be brought to a neutral position or to an active position or to a reverse operational position. According to the German patent No. 1,107,246 this position adjustment is accomplished by exchanging the cam disks, that is by removing one set of disks and inserting another set.
In another known embodiment, such as for example in the DL patent No. 83 576 and in the German Published patent application No. 2,419,747, the cams for the selected functional group are coupled together and thus can be replaced as a pack when the machine is switched over to another operational mode; nonetheless, it is still necessary to readjust additional functional groups of the turnover cylinder depending on the particular arrangement of its cams.
Also in this prior-art solution, the existence of a large number of driving means necessitating separate adjustment of respective functional groups increases the proportion of manual attendance which is further increased due to the locally different arrangement of the drives (at both side disks of the turnover cylinder) and also contributes to an enlarged width of the printing machine.
Due to the large number of necessary adjusting steps, the prior-art printing machines are therefore prone to adjustment errors which may cause degradation of the function and even damage to the printing machine.